This invention relates to heated platens in general and more particularly to an improved heated platen less subject to leaks. Heated platens are used for various functions. Typically platens of this nature are made of aluminum and contain within them a plurality of parallel channels through which a heat exchange fluid may be passed in order to heat the platen. In large platens, the head exchange passages are divided up into zones so that all areas of the platen are equally heated, otherwise, the fluid by the time it went from end of the platen to the other would be considerably cooled and there would be uneven heating.
One type of heated platen that is of particular interest is a platen used in carrying out the expansion of a thermoformable material using a process such as that described in U.S. Pat. No. 3,765,810 and as also described in U.S. Pat. No. 4,113,909 granted to Donald R. Beasly and assigned to the Norfield Corporation, the same assignee as the present invention. In the process disclosed therein, a thermoformable plastic material is disposed between two platens, each of the platens having a surface containing a large number of vented openings. The platens are heated to a temperature of equal to or greater than 70.degree. C. at which point the thermoformable material bonds by hot tack adhesion to the platens at areas other than those containing openings. The platens are moved apart and venting occurs at the openings. Thus, as the platens are pulled apart a cellular structure results as more fully disclosed in the aforementioned patents. The platens are then cooled whereupon the expanded material can be removed from between the platens.
The type of platens which have been used commercially for this purpose contain a plurality of circular holes all of which are vented, the holes being arranged in rows and columns which are staggered. The aforementioned U.S. Pat. No. 4,113,909 describes the manner in which such platens can be supported so as to maintain a planar surface. Although the method and apparatus described therein has been used in commercial applications to produce large panels of for example 4.times.8 feet, various problems have occured with platens.
At the vent areas, vent passages have been previously drilled through the platens in a direction perpendicular to the plane surface of the platen. Only small holes can be used so as to avoid running into the heat exchange channels which contain the heat exchange fluid. As a result, because of the nature of the plastic material being expanded, the holes tended to become clogged. A further problem with platens containing the vent passages in their top surface in that cracks have formed in the area between the heat exchange channel and the vent passages. Such cracks are not accessible, as are cracks which might occur in the bottom of the platen. Thus, the occurence of such cracks, may mean that the whole platen has to be scrapped and a new one made.